Superheterodyne circuit



May 2, 1950 E. E. coMBs SUPERHETERODYNE CIRCUIT Filed Aug. 2, 1945 Nm, 1111 l- QIN DMX-u ...Om .rZOU

INVENTOR.

EDWARD E. COMES mOPUmJmm @262m lml, -Ail o NTI Omg Patented May Z, 195i) ilNlTED STATES PATENT OFFICE SUPERHETERODYNE CIRCUIT Edward E. Combs, Long Branch, N. J.

Application August 2, 1945, Serial No. 608,570

4 Claims.

( Cl. Z50-20) (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) The invention described herein may be manufactured and used by or for the Government for governmental purposes, Without the payment to me of any royalty thereon.

My invention relates to a novel superheterodyne receiver which provides a number of advantages over circuits of conventional design.

The major aspects of a radio receiver that determine its qualities are selectivity, sensitivity, and frequency range. There are other attributes of a receiver which are desirable, but these are dependent to a large extent upon the established limits of the three listed qualities. For instance, the fidelity of reception of an amplitude modulated signal is Within limits dependent upon the selectivity of the receiver, high selectivity reducing the response to high modulating frequencies.

The superheterodyne circuit of today provides to a large degree a measure of control over the three listed qualities, the limits of which are established by the designer. With the superheterodyne circuit, the selection of the intermediate frequency is a compromise based on desired selectivity, image rejection ratio, and gain, in vievv of the requirements for signal frequency range.

A good receiver must have adequate and controlled selectivity, and this property can be obtained quite simply for a given frequency by using a fixed-tuned radio frequency amplier. However, in order to obtain exible tuning and still retain the advantages of the controlled and uniform selectivity of the fixed tuned amplifier, superheterodyne action must be used. Thus, a given signal is received in a tunable circuit tuned to its particular frequency, mixed With the output of a local oscillator in an appropriate circuit to obtain a resultant mixture of frequencies, one of which is the desired intermediate frequency signal. The local oscillator is so arranged that its frequency differs from the received signal, either greater or less than, by an amount equal to the intermediate frequency. To maintain this frequency difference constant over the entire fre-y quency range of the receiver, the local oscillator is usually gang-tuned with the mixer input and some means for tracking the oscillator frequency with the incoming signal frequency is used. In the superheterodyne circuit using variable capacitor tuning, use is made of a series tracking capacitor inserted in series with the tuning capacitor section of the oscillator When the oscillator frequency is higher than the received signal, or in series with the mixer tuning capacitor when the oscillator frequency is lower than the received sigmil.V

It becomes obvious when the action of the superheterodyne circuit is examined that, when the ratio of intermediate frequency to signal frequency becomes as low as 2% of the received signal, the tuned input circuit of the mixer cannot easily discriminate between the signal frequency and its image frequency. This problem of frequency discrimination or image selectivity can be considerably reduced by the use of a tuned radio frequency amplifier ahead of the mixer stage. However, even this expedient fails to do much good when the ratio of intermediate frequency to received frequency becomes as small as 1%. Thus, with these facts in mind, it can be practically stated that the upper frequency limit of a superheterodyne circuit for good image rejection is approximately 20 to 25 times the intermediate frequency when using one stage of preselection or radio frequency amplification ahead of the mixer. In the case where an intermediate frequency of 450 kc. is used, the upper frequency limit is approximately 10,000 kc. for an image rejection ratio of 100 to 1 or better, considering the Q of the usual associated circuits.

It was pointed out above that proper gang tuning requires a series tracking capacitor. A mathematical solution of the tracking problem involves a cubic equation for the tuned circuit containing the series tracking capacitor. From this equation it can be shown that the net result is that the two ganged circuits will track exactly at three frequencies only, Within any particular frequency band being examined. Thus, if the receiver is to have a number of bands in order to cover a wide frequency range, it is obvious that the circuits can be made to track at only three frequencies in each band. Even to obtain this result, a different series tracking capacity must be used in each band.

In considering the case of a superheterodyne receiver that is to cover only a single band of frequencies, it is Well known that, by judicious plate bending of the oscillator capacitor orby proper selection of plate shapes, the oscillator and mixer stages `can be made to track exactly throughout the entire frequency range. If this is done, additional mixer input circuit selectivity may be `used Without causing variations in .selectivity usually caused by mistracking of the ganged circuits. But this expedient cannot be used to obtain exact tracking in all bands of a multiband receiver unless a different tuning capacitor is used for each band..

Quite often, the fact that. it is the I.F. amplier that provides the selectivity is overlooked,

especially in the case where two preselector stages are used in front of the mixer stage. If it is Y assumed that the I.F. amplier has a band pass of kc., at 10 times down, and the two stages of preselection give an image rejection ratio of ten to one at 45 mc., then the selectivity of the tuned LPL-F. .preselector .stages is such. that it has a band pass of,twicer-thefintermediateffrequency, or 900 kc. at ten times down. From this comparison, it is quite obvious that, within practical..

limits, the overall receiver selectivity is improved as the intermediate frequency is lowered-, while, on the other hand, the image rejection ratio Yis improved as the intermediate frequency is raised'.A Thus, for any given coveragefoffthe.rad-ioxircaY quency spectrum, a compromise must be made. as to the exact intermediate frequency to `b"e used; In the usual superheterodynecircuit,.theel-F. circuits are not tunable. of conventional superheterodyne action will show that operation of the signal input circuits.- at' or verynear to the Vinterniediate .frequency will-yield spurious.. responses.. If. thesignal.v inputA circuit werey tunedto .4552 ko.' withaei kc.v I.E. .amplia fier, sum andfdiiference frequencies would be impressedupon the input of said..I.-F. amplifier, said difference frequency. produc-ing..A an f. undesirable 5 kc.- beat in thesecond detector. l Theadvantagesbf good selectivityl provided b'y a low intermediate. frequency amplifier are sometimes. combinedvvith .the advantages of good imagerejection offa relatively high intermediate frequency. amplifier. by using the double. superheterodyne. circuit. In.. this. casey the rst I.-F. amplifier.Y might. be .xedly tuned` `toA fk5.y mc. l and thesecondl-F .amplifier tuned. to. affrequency ofAD ker. Tneiheterodyning of.. the output `of -the first mixerA stage With .a .secondlxednlocal roscillator. producesithe. second .Ie-F; output. ar.- rangement, however, ,has the .disadvantage v that signal.inputcircuits cannot begtunedftoa frequency lower. than ther/highest IIb-F., in .this-case From the.. discussion. given,..it. becomes` appar entv thatv with. conventional. circuits, .it is .impractical to.. design. a.. superheterodyne receiver that covers too. largeia .portion .of the. frequency spectrum, because the lowest-.frequency is deter.- mi-ned .b'ythe intermediate frequency., .while the highest frequency. is .determined by. the. image reject-Ion ratiowhich is', in .tu-rn, afunctionoffthe intermeciate frequency selected; Thusfor-or.- dinary purposes,. the ratio.ofi'thehighestfrequencyjto .the lowest frequency that .can he :used toadvantage in thesignal input circuit is about thirty to one forc'ontinuous frequencycoverage,

lt isalprincipalobject ofmy invention `to vpro-- vide` aj novel multiband.V rcceivenof` the superli'eterodyne type which reduces tracking. problems; hence, bandfchanging problems.

It isa further obiect ofmy inventiontopro.- vide aV novel superheterodyne .receiver having both good'selectivity. and goodimage rejection- Another obiectoirny inventionis. to provide a...novel superheterodyne. receive'nhaving. a con stanti image rejection .ratioiin any.4 given band.'h f

Yet another object of my invention: iste provide a novel,l receiver ofthe double fsuperheterodyne type having extended-frequency. range witha minimumcf. bandfswitching.. l u

In accordance with. my inventi'onfuse .iamad oftwo tunable frequency. changers in Vcascade to provide two intermediatefrequencies, rthe iirst high.er.than.the second, whereby. good .image;re-

Detailed.- examination.:

quency, and good selectivity is provided by the second intermediate frequency.

The rst frequency changer comprises a tunable oscillator and a mixer having a tunable input circuit, which may incorporate one or more stages of radio frequency amplification tunable over.a,...desired frequency. range. Both. the oscillator andf-the input circuitof the'mixer are provided with band-changing means. Contrary to the conventional usage in which the intermediate frequency amplifier is ixedly tuned, the first intermediatefrequency amplifier is Ina-de tunable over a fixed band. The tuning range of said mixer input circuit, said oscillator, and said first I.F.

amplieris'such that, for any given hand, the

ratio of the frequency to which the mixer input circuit'is tuned, with respect to the frequency of the oscillator -is maintained constant. Because ofth'is constant ratio, no series tracking capacitor or other means need be provided in any of these tuned.- circuitstomake thecircuits track, since under this vconditionthe circuits are self-track.-

lllg'; V

The second frequency changerfis coupled 'to the output of the rst IILF. a-mplifleri` Its lopera tion is conventional in that its oscillator istunable oversuch a. range .as to y provide a .second intere mediate frequency which is constant `*and lower than any Afrequency*provided b'yfth'e tunable ,L1-F. amplifier. This constant `I.-l"".`is, in turn,ampli ed .by a second .IL-F, amplifier, :which is .fixdl'y tuned. Because of said constanttfrlequency dif-'-V ference, theoscillator .of the second 1 frequency changer must be provided'with'the vusualltrackr ing means;V but, because this. oscillator is ,tunable over a, fixedfrequeney range regardlessofwhich band the receiver is operating. in, tracking problem is relatively simple.. The output ofthe secondi-Fl amplifier is thendetected in the usual manner. K f A l For a better understanding ofthe. invention, together with otherandfurther objects thereof, referencel is` had tothe following description taken in connection with .theaccompanying drawing, which shows a block diagram of4 my invention. Y l 1.

, The drawing. show a-double superlreterod-yne receiver incorporatinga tuned-circuit` Zii, which mayA include one or. more stages of radio frequency amplification, connected between an Y.ane tenna IB and afirst frequency changer. 3), come prising a tunable localosci-ll'ator. 4i! and a mixer 53. The outputof mixer 5s preferably provides a variable, relatively higl,intermediate frequency to provide good imagerejection.. .This output is amplified by a' frstll.-F..ampliiier emv/nich is tunable over a fixed frequency band, and then applied to a second frequency changer 7B, compris-L inel a second mixer 89 and a second tunable cscllator Qil.- The frequency. range of .second oscillator fili is .such that thecutputof mixer 8B is .a second .intermediate frequency. which is :constant and is,.prei`erably, relatively. low to pirovidefor good selectivity. Thesecond intermedi-ate free quency is then .'further anriplifiedlk 'by-avr fifxedly-` tuned .second 1.-]1". Yamplifier lli!) havingany de,- si-red degree of selectivity asin 1- conventional 'receivers. The outputf amplifier! mi isf thenI ap# pliedto the detector and audioy amplifierl Ii- L Components` 20,- 45', 69 a.iifi,9!l .arey provided with continuoustun-inmeans,l exemplified by variablecondensers 22; 42, 52 l and J- 92; respective; ly, preferably-unicontrollecLbyv a sing-leftuning control element. |20; as indicated;by.broken line jection isyprcvidedbytheiiist-intermediatefre I5 22. Components 2li and ll'are ailsoi provided with band selecting means, exemplified by switches 24 and 44, respectively, both preferably unicontrolled by a single4 band-selector control elementl30, as indicated `by broken line it?. A switch l2 is also provided to selectively connect the antenna to the input oi R.F. amplifier 20 or the input of first I.F. amplifier 6c for purposes hereinafter explained.

The first frequency changer does not provide a constant I.-F, output, as is the usual practice. Instead, the tuning range of the signal input circuit to mixer 50, with respect to the tuning range of oscillator is made such that the ratios of maximum to minimum frequencies of both tuned circuits are equal regardless of the band they are operating in; and that, at any setting in a given band, the incoming signal frequency selected by circuit 20 bears a constant ratio to the frequency generated by oscillator 0i), which may be accomplished by making the law of frequency variation of both tuned circuits similar. The resultant I.F. output of mixer 50 is therefore variable and the first I.F. amplifier 00 is always tuned to accept the output frequency of mixer 50. 'Ihis can be attained, even with the unicontrol feature, Without any tracking means in the tuned circuits of I.F. amplifier 60, by making its tuning characteristic such that the ratio of its maximum to minimum frequencies is equal to that of the other tuned circuits in all wave bands, and that the ratio of any frequency to which it is tuned, with respect to any frequency to which the R.F. amplifier 20 is tuned, is constant for any given wave band.

This feature also permits band-changing of R.F. amplifier 20 and first oscillator 40 without the need for any tracking means. The tuning range of iirst I.F. amplifier 60 can also be made xed, regardless of the frequency band in which components 20 and 40 are operating.

Because the second I.F. amplifier 100 is fixedly tuned, means must be provided to make the output frequency of oscillator track with the output frequency of first I.l'. amplifier 60 to provide a constant frequency difference, but this feature i is not objectionable because the tuning range of second oscillator 90 is fixed, regardless of the band in which the receiver is operating.

To further illustrate the operation of the abovedescribed system, let it be assumed that the frequency range of rst I.F. amplifier 60 is 2 mc. 4 mc., a ration of 2 to 1, and the frequency to which the second I.F. amplifier 100 is iixedly tuned is 450 kc. The tuning range of secondlocal oscillator 90 will therefore be from 2.45 nic-L45 mc. The tuning range of the incoming signal frequency amplifier circuit 20 and the tuning range of oscillator 40 will also bear a 2 to 1 ratio, e. g., circuit 2D may tune from 4 mc.8 mc., and the oscillator l0 from 6 mc.12 mc., so that thc outputs thereof will combine to yield difference frequencies of from 2 rnc-4 mc., to which first I.F. amplifier 50 is tunable. In this manner the incoming signal frequency passed at any time by tuned R.F. amplifier 20 will always bear a constant ratio with respect to the frequency of oscillator 00, for any given Wave-band in which these two components are operating, thereby eliminating the need for any tracking expedients.

Operation in the 2 mc.4 mc. band can be accomaction in this band. Or, the same end can be atiii.

tained by timing R.F. amplifier 20 from 21mm-4 mc. and rendering the first local oscillator 40 inoperative. The number of separate bands to which the first local oscillator 40 is tunable can be further reduced by operating it above the incoming signal frequency in some wave-bands, and below the incoming signal frequency in other wave-bands.

Because the first I.F. amplifier is gang-tuned with the other circuits, it is possible to so designi the circuits that at no time will a harmonic of either local oscillator fall within the pass-banda of the tuned signal circuits.

'Ihe novel circuit above-described possesses a: number of other advantages. It makes possible a receiver having a ratio of maximum to minimum frequency as high as to 1, or more, witlr continuous frequency coverage. In fact, a tuning range of 250 kc. to 80 mc., a ratio of 320 to l, is possible.

By properly proportioning the tuning range of the rst I.F. amplifier 60 and the second oscillator 90, incoming signals having a frequency which is the same or near that of the second I.F. amplifier |00 can be received, making it possible to tune the amplifier 20 to a frequency which is even lower than the second intermediate frequency, e. g., 250 kc. Good image rejection will be obtained for incoming signal frequencies which are two to twenty-five times that of the first intermediate frequency band, i. e., up to approximately 80 mc. Because the ratio of signal. input frequency to first I.F. amplifier frequency is a constant for any particular band, the image rejection ratio will also tend to be constant over a given band.

By utilizing the principles above-described, a single receiver can be built which has a limited number of wave-bands which can be installed as need indicates. Thus, a single mechanism will provide a number of receivers each covering a different portion of the radio frequency spectrum; for example, 250 irc-8000 kc. in five bands for one model, l mc.32 mc. for a second model, and 4 nio-128 mc. for a third model. The following is an example of a six-band receiver designed in accordance with my invention:

Frequency of second I.F. ampliiier |00 is 500 kc. Frequency range of first I.F. amplifier 60 is 1.50 mc.3.25 mc. Frequency ratio of circuits 20, 40, and B0 is 2.15 to 1. Frequency range of second oscillatol1 90 is 2 mc.2.75 mc. Desired frequency range of the receiver is kc.-15 rnc.

The tuned circuits of amplifier 20 have the following wave ranges:

Band 1-150 to 325 kc. Band 2-325 to 700 kc. Band 3--700 to 1500 kc. Band 4-1.5 to 3.25 mc. Band 5-3.25 to 7.00 mc. Band 6-7.00 to 15.00 mc.

The tunedv circuits of first oscilator 40 have the following ranges:

This completes the description of my invention. It will be seen that the provision of a double superheterodyne with a tunable, first intermediate-frequency amplifier which is unicontrolled with the other circuits, and the provision, in all variably tuned circuits ahead of the second mixer,

of constant ratio omininium toiriaximinriife quency-Vinaltwaive-bands ygiiovidesf:the` following desirable' charaeteristics ai. o trackingA means i`s",ne"cessary,for^ the ci`rcuits' precedingftheseccnd mixer, thereby' simpliiying wave-tand changing:

15'; The same 'circuit tracking means ,thesec'- ond oscillatorr is suicientior all wave-bands;

cOnly the: tuned-v circuits preceding the' rst li-F amplifier require' Hand' svvitching'.4

highly' selective, low frequency, second 1;-F." amplifier can loe usedj in conj'uncton with received Lfrequencies up' to1100 lund still retain goodliniage rejection characteristics.

e. Constant image rejection" ratori'nj agiven wlezveeband.

` L'Greatly extendeldfi'equency rarigewitli a minimum of circuit interaction and liendswitching,

Wl'iil'eftlrere has beendescribed Whatfis-:at present; considered;v a preferred; embodiment: of tlie invention, it will' besobvious'-toftnoserskilied in the cuit` that usti'iouschanges andffmodiications, may

be made therein without; departing romxtliefinv'er-ition, and t-fsA aimed-*in the appended claims to cover; alll suoliu changer-,zand modifications as fall within the true spiritand scope-ofthe invention.

I'cla-irn:

i. Amultiebandisuperleter'ody prisingga frrstmixer, acontinu c ly vcircuit coupled to tli'e input of said mixer, e; `contirmcus-ly tunable oscillator coupledto seid mixer, a beat .frequency amulier-counled-to the output oftzsaidpmiX-erend continuouslytunableovei u xed predetermined frequency range; e.conti-nun ous'ly tunzilole heterodyneffrequency changer coupled. to the output oisaidbeat frequencyei plier to derivetherefr-curiA second and. ii ed beat frequency from any sig-nalin seid fixed determined frequency.v range, a beatfrequency ampliiier coupled-to the output of frequency changer.k tuned -Vto saidxed 'beat frequency, and*,loandchangemeans to vary only the tuning range of said continuously: tunable circuit and said .continuously tunable oscillator-,the ratios of tlfie maximum to minimum frequencies. of said continuously .tunable circuitand said continuously tunable oscillator` being constant inail WaveV bands. l K Y A 2. A multi-band,superlieterodyne,receiver com? pricing, a first mixer, a continuouslytunable cirreceiner:com.1

cnarrgenieaneto worryi only-:tue: tuning range` cik saidr continuously tunablecircuitfaiidfsaidi com tinuously;A tunabltoscill'ator, theratios': of` the maximum. to minimum-frequencies of saidv continuously' tirn'aloe"` circuitil and-l *said* continuously tunable oscillator being constantfinv all, wave bands.

3. A multi-band superh'cterodynevf receiver coniprifsiiig` a. istfn-'iixerg ai continuously tunable circuit coupled to theV input of said mixer,v` a continuously tunable oscillator-1 coupled tov-said mix'eif'; a: first intermediate frequency amplifier coupled toftlfre output of said-mixer and continuously tunable! over a fixedlpredeterminedff fre; quency range, a continuously tunablfefheterodyne frequency changer;v coupledeto the: output of said intermediate-frequency ampjlerf toZ derive there; fromY afi-lowerxed-intermediate frequency, a second intermediate frequency: amplier coupled to the output of'E seid frequency changer and tuned to said fixed intermediat'el frequency; and bandckiange mean's'fto vary onlythetun'inglrange of "said continuously` tunablecircuit andi said corrtinuously tunable oscillator, said* continuously tunable' circuit" and' said' continuously tunable oscillator' having. tlie same law of? frequency-Varia'- tion throughout allwav'e bands.

4r A multieba'nd' superheterodynereceiver comprising a first mixer, aivconti-nuously tunable rst circuit coupled to the -ifnputfof'said mixer, al continuouslytunable'. oscillator cou-pled to said mixer, a` second circuit-responsive toetlicY output of. said mixer and continuously tunable-- over a xed predetermined frequency range, accntinuouslyl tunable heterodyne frequency-changer couplied to' tlie output offsaidlsecond cii'cui'tfto-v deje rive? therefromr a fixedbeatr frequency from any signalV in said iizred V predetermined frequency range, aA beatfrequency amplifier' coupled to the output of sai-d1 frequency changerVV and tunedto said ilxed-beatlfrequency', and band change means tovary only-the tuning-range-of'sad'iirst circuit andfsaid continuouslyl tunable oscillator, said ccntiiiuously tunable rst circuit and sai`d-continu ouslytunable oscillatorhavingthe samelaw of frequency vvariation throughout all wave bands.

EDWARD' E; COMES.

REFERENCES'` CITED The; iolloyvingreferences are of record in the file of this patent:

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